A thermal flow meter that measure a flow rate of gas is configured to include an air flow sensing portion for measuring a flow rate, such that a flow rate of the gas is measured by performing heat transfer between the air flow sensing portion and the gas as a measurement target. The flow rate measured by the thermal flow meter is widely used as an important control parameter for various devices. The thermal flow meter is characterized in that a flow rate of gas such as a mass flow rate can be measured with relatively high accuracy, compared to other types of flow meters.
However, it is desirable to further improve the measurement accuracy of the gas flow rate. For example, in a vehicle where an internal combustion engine is mounted, demands for fuel saving or exhaust gas purification are high. In order to satisfy such demands, it is desirable to measure the intake air amount which is a main parameter of the internal combustion engine with high accuracy. The thermal flow meter that measures the intake air amount guided to the internal combustion engine has a bypass passage that takes a part of the intake air amount and an air flow sensing portion arranged in the bypass passage. The air flow sensing portion measures a state of the measurement target gas flowing through the bypass passage by performing heat transfer with the measurement target gas and outputs an electric signal representing the intake air amount guided to the internal combustion engine. This technique is discussed, for example, in JP 2011-252796 A (PTL 1).
JP 2009-85855 A (PTL 2) describes a flow amount measurement apparatus includes a housing having a proximal end portion attached to an intake duct and a leading end portion protruding toward the center of the intake duct. In this flow amount measurement apparatus, a connector terminal is provided at the proximal end portion of the housing, and a flow amount detection device is provided at the leading end portion of the housing. The flow amount detection device is provided on an electric circuit attached to the housing, and an electric circuit and a connector terminal are connected by a bonding wire.
The temperature of the intake duct increases according to the increase of the temperature in the engine room, and the heat is also transmitted to the housing of the flow amount measurement apparatus. The temperature of the proximal end portion of the housing increases because of heat transfer from the intake duct, but the leading end portion of the housing is cooled by the intake air and the temperature thereof decreases, and this causes temperature difference in a direction perpendicular to the air flow direction of the intake duct.
Because of this temperature difference, the housing expands or shrinks in the longitudinal direction, and when there is a change in the distance between the electric circuit and the connector terminal, the bonding wire is deformed, so that the change of the distance therebetween is absorbed.